0.002 to 0.000012 billion years ago
2.0 to 0.012 million years ago
2,000,000 to 12,000 years ago
(WI and IL)
Quaternary (Pleistocene) 0.002 bya (WI and IL)
For the following 360 million years, Wisconsin was a land area that did not accumulate any permanent sedimentary deposits. This is indicated by the lack of strata of late Paleozoic, Mesozoic, and Tertiary age.
About one million years ago, during the Quaternary Period (also called the Ice Age), glaciers intermittently covered all but the southwestern corner of the state. The glaciers and their associated river and lake systems left mud, sand and gravel deposits that cover the PreCambrian and Paleozoic rocks. In places, these deposits contain the bones of Ice-Age mammals such as mastodons and giant bison.